resistance feedback for spot welding does not reliably

Sheet Metal Welding Conference XVIII

Livonia, MI

October 17-18, 2018

RESISTANCE FEEDBACK FOR SPOT WELDING DOES NOT RELIABLY CONTROL THE PROCESS Robert Slazinski RMS Works

Abstract It is claimed that resistance feedback can be used to control the process in the resistance spot welding (RSW) of advanced high-strength steel (AHSS), dual-phase, and other advanced and standard steels. Resistance feedback cannot be reliably used to control the RSW process. Even though the idea of using resistance to attempt to determine weld quality has been tried for over 70 years, the idea persists. Manufacturers of resistance welding equipment rely primarily on lab testing which provides a false sense of success. The industry lacks the documented results of real-world testing that show it is not a reliable, predictable method of controlling the process. This paper provides the history of the ideas of using resistance as a control parameter and how they relate to present day implementations. This paper also describes the history of the present day mathematical modeling that confirms the real-world results. The paper also attempts to address the reasons why resistance feedback technology appears to perform but does not provide predictable results. Introduction It is claimed that using resistance feedback adaptive control of the resistance welding process will result in reduced expulsion while maintaining weld quality. Testing and analysis showed that the normal variation present in the production process would cause the resistance feedback control to either produce discrepant welds (undersized weld nuggets) or increase expulsion. Another claim is that resistance feedback control would provide accurate quality data (weld nugget size). Testing and analysis showed that there was no correlation between any weld control generated weld quality data and actual measured weld nugget size. Investigation into the history of this technology shows that it has been attempted in various forms for over 75 years. Only recently has research and modeling been done to show that there is not enough information present in the electrical data to reliably control or predict the nugget size of RSW. Although detailed lab report testing exists for resistance feedback, these are mostly provided as a sales function of the technology. This paper provides a real-world, long-term comparison of resistance feedback to the standard constant current method of heat control. History of Resistance Feedback RSW is the process of joining sheets of metal by using electrodes to generate pressure and then passing electricity to generate heat to fuse the metal. The three programmed parameters of pressure, heat (electrical current), and time are arrived through extensive empirical testing of each given combination of metal.

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Livonia, MI

October 17-18, 2018

Since electric welding was invented, there has been a desire to make the process automatic. In 1888, 2 years after the granting of a patent for Electric Welding, Elihu Thomson was granted another patent for an Apparatus for Electric Welding.

“My present invention relates to an improvement in electric welding apparatus whereby the welding-current may be automatically cut off from the machine upon the completion of the welding process or the carrying of the welding to a predetermined point. By this improvement the machine becomes practically automatic in its operation…”(1) This novel approach used the physical aspects of the process to provide feedback information. The amount of movement of the clamps holding the pieces to be welded is used to determine when to shut off the current. This method later has been refined to measuring the amount of deflection in the welding electrodes and is still being pursued today. The implementations of adaptive resistance feedback that were investigated use the electrical data measured at the primary of the welding circuit and voltage measurements taken from wires attached at the electrode tips to calculate the dynamic resistance and adjust the voltage output during the weld. One of the earliest references to utilizing this resistance for controlling the process is found in a patent granted to Cletus J. Collum of Weltronic Corporation, applied for in 1939.

“My invention has for its object to produce an efficient means for terminating the flow of the welding current immediately upon the production of a welding integration of the parts of the work such that, when cooled, an efficient weld will be produced. The invention, particularly, provides means that is sufficiently sensitive to the changing resistance to the flow of the welding current, through the parts of the work, to cause termination of the flow of the welding current when a proper welding integration of the metal of the parts of the work is produced to prevent excess fusion or melting of the metal and to prevent burning of the metal.”(2)

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Livonia, MI

October 17-18, 2018

Weltronic Corporation became WTC (Welding Technology Corporation) in 2000. This is one of the vendors whose present implementation of resistance feedback was investigated. This 1939 Weltronic patent describes a method similar to Elihu Thomson’s patent where the welding current is terminated after detecting a pre-determined point. In this method, the current is maintained at a steady rate, and a straightforward resistance drop during the weld is detected to be the terminating point of the welding current. A perceived improvement to this method was patented by Herbert D. Van Sciver II, of The Budd Company, applied for 18 years later in 1957.

“I have discovered that the rate at which the weld nugget is formed is a reliable parameter for control to assure the production of an acceptable weld. Weld nugget formation increasingly shunts the contact resistance between the parts of a workpiece at a rate which depends only upon the rate of growth of the weld nugget. Sensing and control of this resistance change can correct for any disturbances occurring during the welding operation.” “A more specific object of my invention is to constrain the resistance change during the formation of a weld according to a predetermined function which assures the production of an acceptable weld.”(3) This 1957 Budd Company patent is a representative general description of the method employed by present-day resistance weld control manufacturers’ resistance feedback weld controls.

Figure 1. Van Sciver Patent Resistance Diagram

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Livonia, MI

October 17-18, 2018

“Common causes of failure of a welding operation to produce an acceptable weld are illustrated by the dotted extensions, a and b, of the total resistance curve. Curve a is developed when weld nugget growth is so rapid as to cause a “spit,” the extrusion of molten metal between the parts of the workpiece.”(3) The 1957 Budd Company patent describes a method for detecting weld expulsion. Some failings were recognized and an improvement to this system was then described and applied in a patent applied for ten years later in 1967, by Peter W. Vanderhelst, of Robotron Corporation. Robotron was acquired by Weltronic in 1999, which is now WTC (Welding Technology Corporation).

Vanderhelst describes that Van Sciver fails to recognize that electrical noise or transient irregularities occur at the beginning of the weld. “The Van Sciver system would see this difference as an error in the contact resistance and would correct for it by decreasing the weld heat in an attempt to slow the drop of the contact resistance. The effect, therefore, of this initial error is to cause rather than prevent the deviation of weld resistance from its intended decreasing function and to erroneously decrease the amount of weld current during the initial portion of the weld which may affect the weld quality.” (4) It is important to note that the action of decreasing weld heat at the beginning of the weld is designed into both present-day manufacturers of resistance feedback weld controls. During testing this phenomenon occurred causing undersized welds. Although both manufacturers have a method to limit the amount of current reduction, it was not enough to prevent the undersized welds. Information on commercially available products from the past is difficult to find. One system that was available was manufactured by Square D Company in 1962. Above is a section of a brochure describing the system. The full brochure is reproduced in Appendix A. Below is a section of the Bosch PSQ manual from 2014(5) which claims the same features.

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Livonia, MI

October 17-18, 2018

Figure 2. Square D NOR/matic Control Brochure

Figure 3. 2014 Rexroth PSI 6xxx UI Regulation and Monitoring Description of Application

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Livonia, MI

October 17-18, 2018

Increased adaptive functionality is described in a patent applied for 13 years later in 1980 by Erwin E. Denning of Weltronic Company. This is a function similar to a stepper, where the target current is increased specifically to compensate for electrode wear and mushrooming.

“The resistance across the weld electrodes is monitored by the controller during the heat-up phase of the weld and compared to target values for minimum resistance (Rmin) and rate of resistance change (dR/dt). If the observed values for Rmin and dR/dt bear a predetermined relationship to the target values, a dynamic correction is made in the percentage heat control setting. The weld is then continued at the revised weld current level until the desired dR value is satisfied. If upon weld termination, the observed weld time is greater than the target weld time, an additional correction in weld current is made in preparation for the next weld. In addition, the controller is also adapted to detect newly dressed weld electrodes and automatically respond by reverting to the original weld schedule.”(6) This functionality is incorporated into the present-day implementation of the WTC RAFT system. During testing a bug was discovered in this function causing the algorithm to erroneously calculate a maximum weld time. This was corrected in a later version of weld control firmware. A similar functionality was added to the Bosch PSQ UI regulation. The Bosch functionality differs in that the delivered current will not be reduced below the constant current with stepper boost value. This turns the resistance feedback process into a standard constant current with stepper system and provides no additional value. A major departure from using a simple resistance drop for determining current values and weld quality is described in a patent applied for three years later in 1983 by Albert Houchens et al., of General Motors Corporation.

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Livonia, MI

October 17-18, 2018

“A prediction of whether a weld is a nugget or a sticker is made by calculating the ratio of weld energy after the onset of melting to the total weld energy, the ratio of the resistance drop after the resistance peak to the peak resistance, and then a weighted sum of the energy ratio and resistance drop ratio.”(7) The disclosure goes on to describe methods to detect edge welds. No current information is available. This patent technology was not put into use. A more sophisticated approach was taken 14 years later in 1997 in intellectual property as described by James G. Schroth, of General Motors in conjunction with Thomas Zacharia and John Allen, Jr. of Lockheed Martin Corporation doing research at Oak Ridge National Laboratory in Oak Ridge, Tennessee.

“A spot welder comprises a neural network for processing, in real time, current and voltage energizing a weld in progress. The neural network generates a predicted time of optimal weld 7


Livonia, MI

October 17-18, 2018

strength and/or nugget size for the weld in progress. A controller terminates the weld in progress at the predicted time.”(8) This method uses a neural network to analyze the electrical information to determine a given time to terminate the weld current. It does not describe making any current adjustments during the weld to adjust for process variables. In a discussion with Dr. Schroth, this system was tested at a metal fabrication facility. Generally recalling the scenario, the system did not produce consistent results. There was discussion that if more information than just electrical information (pressure, tip condition) were included it would be worthwhile to pursue. Dr. James G. Schroth is still in active development at General Motors Research Laboratory. His recent work includes development of a multi-ringed domed electrode and related processes for the RSW of aluminum sheet metal. Real-World Testing Because of the weld control manufacturer’s provided results of laboratory testing, and the supposed reported success of other production facilities, it was decided to deploy the resistance feedback process wide-spread at an automotive production facility in North America. The primary intent of using resistance feedback was to reduce weld expulsion. No additional staff were assigned to provide dedicated support to the set-up and monitoring of the process. The initial five applications were straightforward as there was only one metal stackup per application. There was some success in reducing weld expulsion, but there were also difficulties with cold welds and sticking caps. The weld control firmware was updated to attempt to address these issues. The resistance feedback process was expanded on an additional six applications. There were more discrepant welds (cold welds, undersized nuggets). It was difficult to pin point the failures, because the adaptive process treats each weld uniquely. Standard discrepant weld containment procedures did not readily address the unique requirements of the resistance feedback process. Real-World Testing – A/B Comparison Because there had been no recent, real-world, detailed comparison of the performance of resistance feedback and standard constant current, it was decided to select an application that would provide the best comparison. An offline station that produced a small sub-assembly met the requirements. This application allowed easy data correlation to the welds produced and access to frequent weld destruct samples to take measurements that would be needed to perform the analysis. This tool is a single-robot, single-transformer (MFDC TRU-79X, 72:1), single-welding gun which produced both a right-hand (RH) and left-hand (LH) component. There are separate fixtures to hold the LH and RH parts. The part consists of two brackets of G1.50mmG welded to a rail of G0.70mmG. One bracket has two weld spots, and the other has four weld spots. LH and RH parts are mirrors of each other. 8


Livonia, MI

October 17-18, 2018

As required by adaptive systems, each spot is fired on its own weld schedule. Schedules 1-6 are on the RH part, and Schedules 7-12 are on the LH part. For comparison, one part was welded in resistance feedback mode, and the other in standard constant-current mode. The goal of the testing was to see if the resistance feedback welding process could provide stable weld quality with as minimal weld expulsion as possible compared to standard constant current. The setup process for resistance feedback welding requires that the application be running to provide consistent weld quality without weld expulsion that can be used as a reference. This requires that the schedule parameters are set, a part is welded, then destructed to measure the weld nuggets. If the nuggets are not the target sizes, adjustments to the schedule parameters are made, another part is welded then destructed. This process is repeated until all weld spots meet the target sizes. A/B Comparison Details - WTC The application was setup using standard production weld schedules (single-pulse constantcurrent) and related parameters. Each weld spot parameters were adjusted to provide the target size nugget. The target size nugget was determined to be 5 mm, which gave a 1-mm tolerance to the minimum required 4-mm nugget size. The LH side was configured to use WTC’s RAFT (Resistance Adaptive Feedback Technology) process. The RH was configured to use standard constant current. The RAFT part produced an overall average of 30% expulsion, and the constant-current part produced 55% expulsion. The RH constant-current schedules were then adjusted to reduce expulsion to 25%, then adjusted further to reduce expulsion to 5%. After running regular production for 1 week, RAFT adaptive was enabled. Destruct weld testing the next day revealed 0-mm nugget size with no fault annunciation. RAFT was turned off. The next weld in constant current produced an acceptable 4-mm nugget. Reviewing the data showed that the resistance curves between the 0 and 4 mm were similar between the reference curve which produced a 5-mm nugget. Data was provided to WTC for evaluation. There was no definitive technical response. The tool then ran both LH and RH in constant current for the next 8 months. No changes to the process or schedule programming were made. The expulsion gradually increased from 5 to 25%. A new tip-dress cutter was installed, and the expulsion reduced to 10%.

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Livonia, MI

October 17-18, 2018

100% 90% 80% Constant Current Parameters adjusted to reduce expulsion

Percent Expulsion

70%

S1-6, RH

S7-12, LH

60% RAFT adaptive attempted on reduced expulsion parameters Produced 0MM nugget welds Both sides returned to constant current

50%

Replaced Tip Dress Cutter

40% 30%

20% 10%

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Figure 4. WTC RAFT/CC Average Expulsion A/B Comparison Details – Bosch The Bosch PSQ-6000 medium-frequency direct-current (MFDC) weld controller was installed on the same application to compare its performance to the WTC and to continue the A/B comparison from resistance feedback (termed UIR), and constant current (termed KSR). The parameters for both LH and RH parts for KSR were adjusted and produced 7% expulsion with no discrepant welds (cold, or undersized weld nuggets). After running regular production in KSR for 2 weeks with no issues, the Bosch US technician turned on UIR adaptive mode on the LH part. Destruct weld nugget measuring during the next tip-dress cycle, undersized (